Such heat exchanger is known from Japanese Patent Application Laid-open No.57-2983. There is also a heat exchanger known from Japanese Patent Application Laid-open No.59-183296, which includes high-temperature fluid passages and low-temperature fluid passages defined alternately between heat-transfer plates disposed in parallel, and outlets and inlets for a high-temperature fluid and a low-temperature fluid, which are defined by cutting opposite ends of each of the heat-transfer plates into angle shapes.
When ducts are connected to the high-temperature fluid passages and the low-temperature fluid passages in a heat exchanger made of a metal, it is necessary to bond ends of a partition plate forming the duct to the heat-transfer plates of the heat exchanger by brazing. The heat exchanger in which the opposite ends of each of the heat-transfer plates are cut into the angle shape, as described in the above Japanese Patent Application Laid-open No.59-183296, suffers from the following problem: The material yield for the heat-transfer plates is naturally poor, and it is necessary to braze the partition plate to the apex of the end surface resulting from the cutting into the angle shape. For this reason, it is difficult to carry out the brazing operation because of a small brazing area, and moreover, it is difficult to provide a sufficient brazing strength.